Method of electrocoating strip



Aug. 11, 1953 J. S. BUS ER METHOD OF ELECTROCOATING STRIP Original FiledJuly 5, 1944 1 x w l INVENTOR.

JOSEPH s. BUSER,DEGEA BYBY ooms E. MORGAN,ADMIN ATTORNEY i atented Aug.

METHOD OF ELECTROCOATING STRIP Joseph S. Buser, deceased, late ofMai-land Heights, W. Va., by Doris E. Morgan (formerly Doris E. Buser),administratrix, Altoona, Pa., assignor to National Steel Corporation, acorporation of Delaware Continuation of application Serial No. 543,318,July 3, 1944. This application March 10, 1949,

Serial No. 80,723

9 Claims.

The present invention relates to the electrolytic coating of a basemetal, and more particularly to the treating of the strip immediatelyafter it has been coated with a suitable metal coating and to therecovery of the electrolyte carried from the plating cells during thecoating operation. The invention is particularly applicable to theelectrolytic manufacture of tin plate and will be described herein asapplied to the manufacture of such a product. However, it will beapparent, as the description of the invention proceeds, that it'is notlimited to the manufacture of such a product but may be utilized inconnection with the coating of various types of base metals with varioustypes of coating metals. This application is a continuation ofapplication Serial No. 543,318, filed July 3, 1944, by Joseph S. Buser,deceased. Application Serial No. 543,318 is now abandoned.

In the manufacture of electro-coated tin plate, the strip-like metal ispreliminarily treated and is then coated by passing it through anelectrolytic bath containing a plurality of anode blocks of tin. Duringits passage through the electrolyte, the strip is the cathode and thetin from the anode blocks is plated on the strip as it passes throughthe electrolyte. In one such system the strip is successively passedthrough a' plurality of plating cells where the strip is plated on oneside and is then passed successively through another group of platingcells where it is coated on the other side. In another system which isin use commercially, the strip is passed vertically downwardly andupwardly through a suitable bath past the anode blocks and is coated onboth sides at the same time. The present invention is applicable toeither of these systems as it relates to the treating and handlin of thestrip after it has been coated.

As the strip leaves the final plating cell, it carries with it anappreciable amount of the electrolyte. The electrolyte employed in anyof these coating or plating systems is quite ex-v pensive and,consequently, it is highly desirable to recover as much of it as ispossible and return it to the plating line. However, in the recovery ofthe electrolyte and the return of it to the plating line several factorsof importance must be taken into consideration. In the firstplace, it isnecessary that the rinsing solution be of such character that it willnot have a dele-.

terious efiect upon the product. And in the secnd place, the rinsingsolution, after it has washed .the electrolyte from the coated strip,.must. be of such.chara.cter that itcanbere-vturned to the plating systemwithout appreciride, potassium chloride, potassium fluoride or ablymodifying the electrolyte from the desired composition. The presentinvention provides a method and apparatus whereby a high proportion ofthe electrolyte carried from the plating cells by the strip can berecovered and returned to the plating line without adversely affectingthe product or the electrolyte in the system. In fact, as will bepointed out hereinafter, the method which this invention provides isdecidedly advantageous from the standpoint of the quality of thefinished product.

Difficulty has heretofore been encountered in obtaining a product havinga bright finish thereon. As a consequence, various proposals have beenmade for eliminating the dull finish of the electro-coated product. Ithas been proposed to burnish the surface by mechanical brushingapparatus. It has also been proposed to subject the coated strip totemperatures sufliciently high to cause the coating thereon to flow andthereby eliminate the dull mat finish. Flow brighteners of severaldifferent types have been proposed, including (1) the passing of thestrip through a heated oil bath and (2) the heating of the stripelectrically to the temperature required for re-fiowing the coating. Ithas been found, however, that the film of electrolyte carried from theplating bath adversely afiects the appearance of the resulting producteven though it is subjected to a flow brightening operation. It has beenfound that if the film of electrolyte is not eliminated from the stripit becomes hydrolyzed to stannous or stannic hydroxide and that duringthe passage of the strip through the flow brightener it is converted tothe oxide and that this results in a chalky appearance in the finalproduct. Accordingly, by the present invention one not only recovers theelectrolyte which is carried fromthe plating cells but also provides forthe elimination of a substantial proportion of the solution which maylater be hydrolyzed to stannic or stannous hydroxide and adverselyaffect the finished product.

Various types of electrolytes may be utilized in the manufacture of tinplate. For example, tin

chloride, tin fluoride, and tin sulphate baths have been utilizedheretofore. The electrolyte may contain other salts of the "acidconstituent employed, such as sodium chloride, sodium fiuopotassiumsulphate. Where a tin chloride or tin fluoride bath is employed, thesolution car rierl from the plating cell includes stannous ride, or boththe stannous and stannic chlorides or fluorides. Where a tin sulphatebath is employed, the solution carried from the plating cellsincludes-stannous sulphate or stannic sulphate, or both.

Where the solution carried from the plating cells includes stannouschloride or stannic chloride, or both, in the absence ofproper treatmentthe stannic chloride will hydrolyze to stannic hydroxide and thestanneus chloride will hydrolyze to stannous hydroxide. Similarhydrolysis takes place where t'in sulphate is present in theelectrolyte; that is, the stannous and stannic sulphates hydrolyze tostannous and stannic hydroxides. The presence of these hydroxides isobjectionable in view of the effect on the resulting product.Consequently, it has been found that in recovering the solution which iscarried from the plating cells it is desirable to prevent the formationof the objectionable stannous hydroxide or stannic hydroxide on thestrip to any substantial extent. Whereuntreated water is used in therecovery process, an excess of these objectionable compounds is formedand must be removed from the strip before it passes to the reflowingapparatus. It has been found, however, that by utilizing a propersolution for rinsing the strip a substantial proportion of the solutioncarried from the plating cells can be recovered without the formation ofan excessive amount of stannous or stannic hydroxides.

In the operation of a line for the electro-coating of strip, the bath isusually maintained at an elevated temperature, for example, in theneighborhood of 135 F. Considerable evaporation takes place during theoperation of the line, and, consequently, provision must be made forreplacing the evaporated solution. It has been found that this can beaccomplished by supplying to the rinsing apparatus an amount of waterapproximately equal to the evaporation losses and collecting the rinsewater and returning it to the plating cells.

In the accompanying drawing there is shown, for purpose of illustrationonly, apparatus which may be used in carrying out th present invention.However, the invention is not limited to the use of any specificapparatus as it may be otherwise practiced or embodied without departingfrom the scope thereof. As shown in the drawing, which is diagrammaticonly, the strip S passes through the final plating cell 2 and betweenthe contact rolls 3. It is then carried past sprays 4 which spray on thestrip a rinsing solution from the recovery tank 5, the nature of whichwill be described hereinafter. The strip is then passed between squeezerollers 6. It is then again sprayed by sprays l with the rinsingsolution from the recovery tank. It then passes between squeeze rollers8. It again passes between sprays 9 which spray solution from therecovery tank on it, and it is then passed between squeeze rollers 10adjacent the recovery tank. During the passage of the strip between thealternately arranged sprays and squeeze rollers, it is located above acollecting tank H in which the solution sprayed onto the strip by thesprays and squeezed from the strip by the squeeze rolls is collected.The alternate spraying of the rinsing solution and squeezing thesolution from the strip results in the elimination of a substantialproportion of the solution carried from the plating cells.- Theconcentration of tin chloride in the electrolyte is approximately 4ounces per gallon. The first set of sprays 4 dilutes this toapproximately 1 ounce per gallon. The succeeding sprays and squeezerolls -further dilute the-solutioncarried by the strip from the platingcells, so that at the time the strip enters the recovery tank theconcentration of tin chloride is down to approximately .3 ounce pergallon.

After alternately passing between the sprays and squeeze rolls, thestrip is passed into and through the recovery tank 5, which contains therinse solution, where it is further rinsed. The strip then passes fromthe tank, between squeeze rolls l2, and then over a tank l3 where it issprayed with cold water by means of sprays I l. The strip then passeseither to the hot rinse described and claimed in the copendingapplication Serial No. 546,091 or to drying apparatus and then to thereflowing apparatus, in the event that the coated strip is to besubjected to a reflowing operation.

The rinsing solution contained in the recovery tank 5 and which issprayed onto the strip by the sprays 4, l and 9 water which has beentreated by the addition of a salt or an acid which will lower the pHvalue of the water to a point where objectionable hydrolysis of the tinsalt carried from the plating cell is eliminated. Where an acid is addedto the rinse water, the pH value of the rinsing solution should bemaintained at approximately 1.90 or under. However, where salts areemployed, it has been found that the pH value may be somewhat higher,for example, approximately 2.5, without substantial hydrolysis takingplace and without adversely afiecting the electrolyte solution when therinsing solution is returned to the system.

Where the solution carried from the plating cells includes stannouschloride or stannic chloride, it has been found that excellent resultscan be obtained by utilizing a rinse water which has been treated withsodium fluoride, .sodium bifluoride or hydrofluoric acid. Hydrochloricacid also may be used, but it has a definite tendency to etch the tincoating, and hence it has been found preferable to utilize hydrofluoricacid instead. Various other salts of hydrofluoric and hydrochloric acidsmay be used so long as the pH value of the rinse water is lowered to asuitable extent and so long as they will not have a deleterious effectupon the final product.

Where the solution carried from the plating cells contains stannous orstannie sulphates, sulphuric acid, phenol disulphonic acid andvarioussalts of these acids may be used to bring the pH value of the rins waterdownto therequired value.

Where the electrolyte contains tin chloride, the pH of the bath isordinarily maintained between approximately 2.5 and 3.5. Therefore, theuse of large quantities of hydrofluoric acid in the rinse water woulddeleteriously afiect the pH of the electroplating bath since the rinsewater is returned to the electroplating bath. For this reason, it ispreferable to use a salt such as sodium fluoride or sodium bifluorid inthe rinse water instead of the acid since a higher pH value in the rinsewater can be maintained without hydrolysis taking place where the saltinstead of the acid is used.

The Water for the rinsing spray may be fed' to the recovery tank throughan inlet 1-5. The treating agent may be added to thewater before itenters the recovery tank .or it may be added directly to the tank. Freshwater is supplied to the recovery tank at the rate necessary to replacethe rinsing solution fed to the sprays. Since the rinsing solution isreturned :to the plating system in the manner described below, the rateat which fresh water issupplicd to thcrecovery tank is maintained at theamount required to replace the lossdue to evaporation of the platingbath and strip carry-out. The rinsing solution is fed to the sprays A, 1and 9 from the outlet [6. The rinsing solution which is sprayed onto thestrip and a high percentage of the solution carried fromthe platingcells are collected in the collecting tank H, which is located below thesprays and the squeeze rolls.

collecting tank H has an outlet 11 which is connected by suitable pipingto one of the cells in the plating line. Instead of being connecteddirectly to the plating cells, the outlet ll may be connected to thesupply tank for the cells. The rinsing solution returned to the platingsystem is fed to the system at a suflicient rate to take care of theloss of the plating solution occasioned by evaporation andbythe strippassing therethrough.

As stated above, after the strip passes through the recovery tank, it issprayed with cold water by the sprays I4. This rinse water is not fedinto the electrolytic circulatory system but is discharged to the sewerthrough an outlet [8.

The final rinsing of the strip with cold water hydrolyzes any stannousor stannic chlorides or sulphates remaining on the strip, but, due tothe fact that the major portion of the stannic or stannous chlorides orsulphates has been eliminated from the strip, the adverse effects of thehydrolyzing .of the small remaining amount of stannous or stannic saltscan be eliminated by subjecting the strip to a suitable hot rinsingspray, such as is described and claimed in the copending applicationreferred to above, prior to the time that the strip passes to the flowbrightener.

From the above description, it will be apparent that the invention hasprovided a method and apparatus whereby the solution carried from theplating line by the strip can, in large measure, be recovered and thatthis can be accomplished without deleterious results to the finalproduct and without deleteriously affecting the electroplating bath bylowering the pH value of the rinsing bath to the point where very littleor no hydrolysis of the tin salts takes place during the recoveryoperation. The recovery of the solution carried from the plating line bythe strip effects a substantial saving in operating costs and, asstated, this can be accomplished without adversely affecting the finalproduct.

While the invention is described as applied to the manufacture of tinplate and while having referred only to electrolytic baths of thechloride or sulphate types, it will be understood that the invention maybe utilized with other electrolytic baths and in the manufacture ofother coated products.

What is claimed is:

1. The method of continuously and progressively manufacturingelectro-tinplated strip comprising the steps of passing the stripthrough an electrolytic cell containing an acid plating bath of aqueouselectrolyte solution including tin salt and fluorine ions, a source ofthe fluorine ions being a tin-free acidic fluorine compound present as acomponent of the electrolyte solution, electrodepositing a layer of tinon the strip while the strip is in contact with the electrolytesolution, thereafter passing the strip from the plating bath with tinsalt-containing electrolyte solution carried from the plating bath onthe strip, rinsing the strip with an aqueous rinse solution obtained byadmixing water and a tin-free acidic fluorine compound, the. amountoftin-free acidic fluorine compound in the rinse solution beingsufficient to .reduce the pH value of the aqueous rinse solution to thepoint where hydrolysis of, the tin salt carried from the plating bath onthe strip is substantially prevented, thereby removing tin salt from thestrip, collecting the rinse solution containing the tin free acidicfluorine compound and the tin salt-con taining electrolyte solutionremoved from,the strip, and supplying the collected rinse solution tothe plating system. 7

2. The method of continuously and progrfis sively manufacturingelectro-tinplated strip, as set forth in claim 1 wherein said addedtinfree acidic fluorine compound is sodium fluoride.

3. The method of continuously and progressively manufacturingelectro-tinplated strip as set forth in claim 1 wherein said added tinfree acidic fluorine compound is sodium bifluoride, I

4. The method of continuously and progressively manufacturingelectro-coated strip as set forth in claim 1 wherein said added tin-freeacidic fluorine compound is hydrofluoric acid.

5. The method of continuously and progressively manufacturingelectro-tinplated strip comprising the steps of passing the stripthrough an electrolytic cell containing an acid plating bath of aqueouselectrolyte solution including tin salt and fluorine ions, a source ofthe fluorine ions being a tin-free acidic fluorine compound present as acomponent of the electrolyte solution, electrodepositing a layer of tinon the strip while the strip is in contact with the electrolytesolution, thereafter passing the strip from the plating bath with tinsalt-containing electrolyte solution carried from the plating bath onthe strip, rinsing the strip with an aqueous rinse solution obtained byadmixing water and a sodium salt of hydrofluoric acid, the amount ofsaid sodium salt of hydrofluoric acid in the rinse solution beingsuflicient to reduce the pH value of the aqueous rinse solution to apoint where hydrolysis of the tin salt carried from the plating bath onthe strip is substantially prevented, thereby removing tin salt from thestrip, collecting the rinse solution containing the sodium salt ofhydrofluoric acid and containing the tin salt-containing electrolytesolution removed from the strip, and supplying the collected rinsesolution to the plating system.

6. The method of continuously and progressively manufacturingelectro-tinplated strip as set forth in claim 5 wherein said addedsodium salt of hydrofluoric acid is sodium fluoride.

7. The method of continuously and progressively manufacturingelectro-tinplated strip as set forth in claim 5 wherein the added sodiumsalt of hydrofluoric acid is sodium bifluoride.

8. The method of continuously and progressively manufacturingelectro-tinplated strip comprising the steps of passing the stripthrough an electrolytic cell containing an acid plating bath of aqueouselectrolyte solution including tin salt and fluorine ions, a source ofthe fluorine ions being a tin-free acidic fluorine compound present as acomponent of the electrolyte solution, electrodepositing a layer of tinon the strip while the strip is in contact with the electrolytesolution, thereafter passing the strip from the plating bath with tinsalt-containing electrolyte solution carried from the plating bath onthe strip, rinsing the strip with an aqueous rinse solution obtained byadmixing water and a; tin free acidic fluorine compound, the amount oftin-free acidic fluorine compound-being suificient toreduce the'pH valueof the aqueous rinse solution to a point where hydrolysis of the tinsaltcarried from the plating bath on the strip is substantiallyprevented, thereby removing tin salt fromthe-strip, collecting the rinsesolution containing the tin-free acidic fluorine compound and the tinsalt removed from the strip, and suppIying'the-coIlected "rinse solutionto the plating system at a rate to replace the solution lost from thesystem by evaporation and by carry-out by the strip.

-9. The method of continuously and progressively manufacturingelectro-tinplated strip comprising the steps of passing the stripthrough an electrolytic cell containing an acid plating bath of aqueouselectrolyte solution including tin salt and fluorine ions, asource ofthe fluorineions Iceing a tin-free acidic fluorine compound present as acomponent of the electrolyte solution, electrodepositing a layer of tinon the strip while the strip isin contact with theelectrolyte solution,thereafter passing the strip from the plating bath with tinsalt-containing electrolyte solution carried from the plating bath onthe strip and rinsing the strip with an aqueous rinse'solution obtainedicy-admixing water and a tin-free acidic fluorine compound, the amountof tin free acidic fluorine compound in the rinse solution being theaqueous rinse solution applied to the strip so that the collected rinsesolution supplied to the plating system replaces the amount of solutionlost from the plating system Icy-evaporation and carry-out by the-strip.

.IDORIS E. MORGAN,

Formerly Doris E. Buser, administmt'rix of the estate of Joseph S.Buser, deceased.

References Cited in thefile of this patent UNITED STATES PATENTS Number'Name Date 2,330,609 Nachtman Sept. 28, 1943 2,372,599 Nachtman Mar. 27,1945 2,407,579 Schweiker Sept. 10,1946

FOREIGN PATENTS Number Country Date 546,269 Great Britain n July 3,1942

1. THE METHOD OF CONTINUOUSLY AND PROGRESSIVELY MANUFACTURINGELECTRO-TINPLATED STRIP COMPRISING THE STEPS OF PASSING THE STRIPTHROUGH AN ELECTROLYTIC CELL CONTAINING AN ACID PLATING BATH OF AQUEOUSELECTROLYTE SOLUTION INCLUDING TIN SALT AND FLUORINE IONS, A SOURCE OFTHE FLUORINE IONS BEING A TIN-FREE ACIDIC FLUORINE COMPOUND PRESENT AS ACOMPONENT OF THE ELECTROLYTE SOLUTION, ELECTRODEPOSITING A LAYER OF TINON THE STRIP WHILE THE STRIP IS IN CONTACT WITH THE ELECTROLYTESOLUTION, THEREAFTER PASSING THE STRIP FROM THE PLATING BATH WITH TINSALT-CONTAINING ELECTROLYTE SOLUTION CARRIED FROM THE PLATING BATH ONTHE STRIP, RINSING THE STRIP WITH AN AQUEOUS RINSE SOLUTION OBTAINED BYADMIXING WATER AND A TIN-FREE ACIDIC FLUORINE COMPOUND, THE AMOUNT OFTIN-FREE ACIDIC FLUORINE COMPOUND IN THE RINSE SOLUTION BEING SUFFICIENTTO REDUCE THE PH VALUE OF THE AQUEOUS RINSE SOLUTION TO THE POINT WHEREHYDROLYSIS OF THE TIN SALT CARRIED FROM THE PLATING BATH ON THE STRIP ISSUBSTANTIALLY PREVENTED, THEREBY REMOVING TIN SALT FROM THE STRIP,COLLECTING THE RINSE SOLUTION CONTAINING THE TIN FREE ACIDIC FLUORINECOMPOUND AND THE TIN SALT-CONTAINING ELECTROLYTE SOLUTION REMOVED FROMTHE STRIP, AND SUPPLYING THE COLLECTED RINSE SOLUTION TO THE PLATINGSYSTEM.